Standard erasable and write once (WORM) optical disks may be formatted with several thousand spare sectors per surface. The purpose of the spare sectors is to provide locations for recording data when defects or deterioration in the media, debris on the media or lens, or intermittent or marginal hardware problems create the inability to reliably read and write data. In optical disk technology, the marks that are placed on a disk when data is written may be as small as one micron; therefore a small defect or a dust particle on the disk can introduce an error when reading or writing the data.
Defects in optical disks are generally of two types. One type is within the structure of the disk itself, is caused during manufacturing and is referred to as a permanent defect. Erasable magneto-optical (MO) disks are more prone to such permanent defects than many other types of media since MO disks are produced with layers of reactive materials such as terbium, iron and cobalt. These active layers are covered with transparent plastic material designed to be impermeable to oxygen and moisture which can cause a permanent defect if they reach an active layer. Such defects typically occur during the manufacturing process but can also arise later during use of the disk if moisture or oxygen reach the active layers during use. Moreover, such defects tend to grow over time. Consequently, if a defect is initially present in an active layer such that only one bit of data in one sector is affected, that defect can grow over time to affect additional bits of data in that sector or adjacent sectors.
A second type of defect creating problems in reading and writing data is contamination due to dust or debris and is referred to as a temporary defect. When a large enough particle of dust lights upon the surface of a disk or drive lens, the laser beam is diffused or blocked and the disk cannot be reliably written or read, if at all, at that particular location. Temporary defects created by a dust particle generally do not grow in size but can move across the surface over time as the dust particle moves. Such defects can also, in many instances, be removed by cleaning the disk and/or lens surface(s).
Intermittent and marginal hardware problems can also cause data access to be unreliable. For example, an intermittent circuit fault in the write channel of the drive system can generate faulty data which is then stored on the disk. Error correction code (ECC) may be able to correct the data but it may not necessarily meet a very high criteria for data correction and therefore be deemed unreliable. Similarly, a low output signal from the read channel could also make the data appear marginal even though the ECC might be able to correct the data.
The effect of a defect on a disk can be the same whether it be due to a permanent defect in a active layer or a temporary dust or hardware defect. Conventional optical disk systems can only detect the presence of an error but are unable to determine the cause. Moreover, even when detected, an error can make it difficult or impossible to correct data that is written on the disk through error correction codes. Therefore, it may be necessary for the data to be moved to a dedicated area on the disk containing spare sectors and the original sector marked as defective. If a spare sector is also defective or contaminated by dust, it becomes necessary to move the data to still another spare sector. It is evident that dust contamination and media defects can rapidly consume spare sectors. It is also evident that, with only a limited number of spare sectors, and with each error requiring the use of one or more spare sectors, spare sectors can be quickly exhausted. Clearly, an optical disk can be rendered unreliable or useless if all, or even most, of the spare sectors have been filled.